3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
FIG. 1a illustrates a radio access network with an RBS 101 that serves a UE 103 located within the RBS's geographical area of service, called a cell 105. In UMTS, a Radio Network Controller (RNC) 106 controls the RBS 101 and other neighboring RBSs, and is, among other things, in charge of management of radio resources in cells for which the RNC is responsible. The RNC is in turn also connected to the core network. In GSM, the node controlling the RBS 101 is called a Base Station Controller (BSC) 106. FIG. 1b illustrates a radio access network in an LTE system. An eNB 101a serves a UE 103 located within the RBS's geographical area of service or the cell 105a. The eNB 101a is directly connected to the core network. The eNB 101a is also connected via an X2 interface to a neighboring eNB 101b serving another cell 105b. 
A radio access network, such as an E-UTRAN or an UTRAN, is often deployed on multiple carrier frequencies. A carrier frequency is the centre frequency used for the radio communication between the RBS and the UE. Carrier frequencies are usually gathered in radio frequency bands of a certain width, typically a couple of 10 MHz wide, defined for usage with a particular Radio Access Technology (RAT). The multiple carrier frequencies may each be allocated in different frequency bands, or within a same frequency band. An RBS may provide a number of radio cells on each carrier frequency, pointing in different directions from the RBS.
FIG. 2a schematically illustrates a deployment of a radio access network on three carrier frequencies A, B and C. Cells on the different frequencies may overlap and cover the same area. A UE 203 within an overlapping area may access the network via a cell on either one of the overlapping frequencies. UEs roaming in the network are moving between neighboring cells in order to stay in contact with the network. UE mobility is possible, both between cells of the same carrier frequency, also known as intra-frequency mobility, and between cells of different carrier frequencies, also known as inter-frequency mobility.
The network is usually in control of the mobility of UEs that are in connected mode. The term connected mode is used to denote the state of UEs with an active connection to the network, such as a Radio Resource Control (RRC) state RRC_CONNECTED in E-UTRAN or the CELL_DCH state in UTRAN. The network performs a handover of a UE in connected mode when the UE is moving between cells. An intra-frequency handover moves the UE from one cell to another of the same carrier frequency. An inter-frequency handover moves the UE between cells of different carrier frequencies.
UEs which do not have an active connection to the network are in idle mode. One example of an idle mode state is RRC_IDLE state in E-UTRAN. Idle mode UEs autonomously select a suitable cell when entering the network and may then reselect another cell at any time, based on the cell reselection rules defined for the particular technology. For E-UTRAN, the rules for cell reselection are specified in 3GPP TS 36.304 V10.1.0, section 5.2.
Due to the random nature of the UE mobility, it is often difficult to predict how UEs are distributed between the different carrier frequencies in the network. Coverage gaps and differences in radio propagation conditions on different frequencies may cause UEs to gather in cells of one carrier frequency and not of other carrier frequencies.
It is also a fact that different cells and the different carrier frequencies may offer system capacity that varies within a wide range. For instance, in E-UTRAN the spectrum bandwidth of each carrier frequency may vary in steps between 1.2 MHz and 20 MHz. The cell configuration, the presence of radio interference, and the distribution of UEs within the cell affecting the so called near-far-relations, are examples of other factors influencing the system capacity.
The distribution of UEs within the network can be influenced in different ways. The network may apply handover in order to handle the mobility of UEs in connected mode. For UEs in idle mode, the network uses system information to influence the cell reselection behavior of the UEs. System information is transmitted in each cell and provides the UE with information about eligible carrier frequencies in the neighborhood and with parameters to control the cell selection and cell reselection of the UEs. The system information transmitted in an E-UTRAN cell is specified in 3GPP TS 36.331 V10.1.0, sections 5.2, 6.2.2 and 6.3.1.
In an E-UTRAN, the cell reselection of the UEs is governed by a few main principles:                1. Cell ranking: The UE ranks the cells based on a measurement of a signal strength, the Reference Symbol Receive Power (RSRP), or of a signal quality, Reference Symbol Receive Quality (RSRQ). The UE reselects the cell with the highest rank. Cell ranking is applied between cells located on the same carrier frequency, i.e. intra-frequency cell reselection, and between cells located on different carrier frequencies with equal priority, i.e. equal priority inter-frequency cell reselection (see bullet 2 below).        2. Absolute cell reselection priority: The reselection between cells located on different carrier frequencies, i.e. inter-frequency cell reselection, and between cells located in different Radio Access Technologies (RAT), i.e. inter-RAT cell reselection, is governed by absolute cell reselection priorities. The UE reselects a cell on the carrier frequency or RAT with the highest priority and with a sufficient reception strength or quality (RSRP or RSRQ).        
An RBS in an E-UTRAN, i.e. an eNB, broadcasts system information to influence the cell reselection the UEs perform. A few different types of System Information Blocks (SIBs) are sent out in each cell, providing the UE with information about the alternative E-UTRAN carrier frequencies and about carrier frequencies in other RATs, with the absolute cell reselection priorities, and with a set of parameters to influence cell ranking and to determine the required signal strength and/or quality for reselection of a cell on the various carrier frequencies in E-UTRAN and/or other RATs.
Using the cell reselection priorities, it is possible for the operator to arrange a hierarchy of the different carrier frequencies, where the UEs favor the frequencies with higher priority. For instance, assigning a high priority to E-UTRAN carrier frequencies and a lower priority to other RATs makes the E-UTRAN capable UEs in idle mode to camp on E-UTRAN frequencies where there is E-UTRAN coverage. It is also possible to give priority to E-UTRAN carrier frequencies with inherent high capacity, e.g., a carrier frequency with a bandwidth of 20 MHz, whereas E-UTRAN frequencies with less capacity, e.g., 5 MHz bandwidth, are given lower priority. In this way, the UEs tend to camp on the frequency providing the highest capacity.
When a frequency hierarchy is not desired, the operator may assign the same priority to a number of carrier frequencies. In this way, UEs tend to camp on the carrier frequency providing the best reception, e.g. in terms of RSRP or RSRQ, according to the cell ranking criteria.
In order to achieve good system and end user performance, it is beneficial if UEs are distributed between the available frequencies in a way such that the capacity the network can offer is utilized in a good manner. From a traffic load balancing point of view, neither a hierarchy of carrier frequencies nor the ranking criteria is necessarily the best option.
In a hierarchical deployment, the UEs tend to gather at the carrier frequency with the highest priority if the coverage is similar. As a result, carrier frequencies with low priority might be depleted or underutilized, while other carrier frequencies tend to be overloaded.
In order to equalize the traffic load, the network may have to handover a number of UEs in connected mode towards the low priority carrier frequencies. However, when the UEs are subsequently released to idle mode, they tend to return to a high priority carrier in line with the cell reselection priorities. Therefore, the same procedure has to be repeated over and over again.
The actions required to keep the traffic load balance thus becomes a burden on the network, especially if the network capacity at the low priority carrier frequency is similar to the network capacity at the high priority carrier frequency.
One way to avoid the recurring need for handover in a hierarchical deployment is to “shrink” the high priority cells to allow for load balancing also in idle mode. This can be achieved with an increase of the required signal strength or quality for UEs in those cells. In this way, UEs at the cell edge do not qualify and have to reselect a cell on a carrier frequency of lower priority. This method could be suitable if the coverage of the high priority frequency is naturally more restricted than the coverage of the low priority frequency. This may be the case, for instance, when there are different radio propagation properties in different frequency bands. If such natural differences do not exist the method is not always suitable, because it causes an asymmetric distribution of the UEs within the cells, with most of the UEs at the high priority frequency gathered close to the cell center and the UEs at the low priority frequency deferred towards the cell edge. Asymmetry of this kind may be unfavorable for the overall system capacity and performance.
In a flat deployment, where equal priorities are used, the distribution of UEs between the carrier frequencies is more random. The UEs perform ranking of the cells located on these frequencies, based on the received signal strength and quality, and reselect cells based on that. The ranking of cells on the same carrier frequency is usually a quite adequate basis for the cell reselection, as it allows the UE to move along the carrier and constantly camp on the currently best cell. The best cell should stand out clearly in the ranking, except for the case when the UE is in a typically narrow region close to the cell edge.
However, the ranking of cells on adjacent carriers is more difficult, especially if the cells on multiple frequencies, possibly in the same frequency band, are provided from the same RBS. This situation is not unusual, and the difference in cell ranking criteria can be very subtle. It may furthermore be difficult to tune the ranking criteria to achieve the desired traffic load balance between overlapping cells.
Another obstacle to providing an optimum distribution of UEs to carrier frequencies is that the cell ranking criteria discussed above does not necessarily reflect the desired criteria for traffic load balancing. The RSRP measure does not take the traffic load in a cell into account. The RSRQ measure is sensitive to traffic load, but not necessarily in proportion to the load balancing criteria. Even a small imbalance of the ranking criteria could cause a serious tilt of UE distribution between the cells.